Plasticity, life history and inclusive fitness: an evolutionary demography perspective on individual variation in fertility and fertility preferences in contemporary Britain
This thesis consists of three papers that explore variation in individual fertility and fertility
preference. The setting for all three papers is the contemporary UK, though the conclusions
have utility for a general understanding of human fertility. All three papers are motivated
by theories arising from evolutionary biology, principally inclusive fitness theory and life
history theory.
The first two papers investigate actualised fertility and whether patterns of fertility in
contemporary Britain are consistent with inclusive fitness theory. Both papers conduct
secondary data analysis of the British Household Panel Study. Inclusive fitness theory
predicts that because relatives share genes an individual may obtain fitness benefits by
increasing the reproduction of a relative. Results support this hypothesis showing that for
contemporary British women kin having more opportunities to influence reproductive
decision-making is associated with pro-fitness fertility outcomes. In the first paper I find
kin accelerate the transition to first birth, and the second paper shows kin also accelerate
the transition to second birth.
The final paper tests a different hypothesis derived from evolutionary theory. Life history
theory predicts that reproductive strategy should have ‘plasticity’ and be liable to alter as
perceived environmental risk changes. This paper uses primary data collected from
University students using an internet experiment and finds that priming respondents using
preceding questions on mortality does alter reported fertility preferences, though the effects
depend upon the priming, fertility preference measure and the sex of the respondent. The
paper also has methodological relevance as it demonstrates the potential for ‘context
effects’ from preceding questions to influence the reporting of fertility preferences.
All three papers present evidence that the incorporation of theories from evolutionary
biology have utility in the understanding of contemporary fertility patterns and processes
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